Polyester resin compositions for calendering

ABSTRACT

A polyester resin composition is calendered to produce a film or a sheet. The polyester resin composition is a polyester having a crystallization half time from a molten state of at least 5 minutes combined with an additive for preventing sticking of the polyester to calendering rolls.

This application is a divisional, of application Ser. No. 09/258,365,filed Feb. 26, 1999, which in turn claims priority to U.S. ProvisionalApplication No. 60/078,290, filed Mar. 17, 1998.

TECHNICAL FIELD OF THE INVENTION

This invention relates to the manufacture of film and sheet utilizingcalendering processes, and more particularly to the use of polyesters insuch calendering processes.

BACKGROUND OF THE INVENTION

Calendering is an economic and highly efficient means to produce filmand sheet from plastics such as plasticized and rigid poly (vinylchloride) (PVC) compositions. The films and sheets usually have athickness ranging from about 2 mils (0.05 mm) to about 45 mils (1.14mm). They are readily thermoformed into various shapes and are used fora wide variety of packaging applications. Calendered PVC film or sheetcan be used in a wide range of applications including pool liners,graphic arts, transaction cards, security cards, veneers, wallcoverings, book bindings, folders, floor tiles and products which areprinted or decorated or laminated in a secondary operation.

Japan Application No. Heisei 7-197213 (1995) to E. Nishimura et al. andEuropean Patent Application 0 744 439 A1 (1996) to Y. Azuma et al.disclose the state of the art with regard to polypropylene resincompositions used in calendering processes.

In a typical calendering process line, the plastic resin is blended withspecific ingredients such as stabilizers to prevent thermal degradation;modifiers for clarity, heat stability or opacity characteristics;pigments; lubricants and processing aids; anti-static agents; UVinhibitors; and flame retardants. The mixed ingredients are plasticizedin a kneader or extruder. Through heat, shear and pressure, the drypowders are fused to form a homogeneous, molten material. The extruderfeeds the molten material in a continuous process to the top of thecalendering section of the calendering line in between first and secondheated calender rolls. Typically, four rolls are used to form three nipsor gaps. The rolls are configured in an “L” shape or an inverted “L”shape. The rolls vary in size to accommodate different film widths. Therolls have separate temperature and speed controls. The materialproceeds through the nip between the first two rolls, referred to as thefeed nip. The rolls rotate in opposite directions to help spread thematerial across the width of the rolls. The material winds between thefirst and second, second and third, third and fourth rolls, etc. The gapbetween rolls decreases in thickness between each of the rolls so thatthe material is thinned between the sets of rolls as it proceeds. Afterpassing through the calender section, the material moves through anotherseries of rolls where it is stretched and gradually cooled forming afilm or sheet. The cooled material is then wound into master rolls.General descriptions of calendering processes are disclosed in JimButschli, Packaging World, p. 26-28, June 1997 and W. V. Titow, PVCTechnology, 4^(th) Edition, pp 803-848 (1984), Elsevier Publishing Co.,both incorporated herein by reference.

Although PVC compositions are by far the largest segment of thecalendered film and sheet business, small amounts of other thermoplasticpolymers such as thermoplastic rubbers, certain polyurethanes,talc-filled polypropylene, acrylonitrile/butadiene/styrene terpolymers(ABS resins) and chlorinated polyethylene are sometimes processed bycalendering methods. Attempts to calender polyester polymers such aspoly(ethylene terephthalate) (PET) or poly(1,4-butylene terephthalate)(PBT) have not been successful. For example, PET polymers with inherentviscosity values of about 0.6 dL/g have insufficient melt strength toperform properly on the calendering rolls. Also when the polyester isfed to the rolls at typical processing temperatures of 160° C. to 180°C., the PET polymer crystallizes causing a non-homogeneous mass which isunsuitable for further processing. The non-homogeneous mass causesundesirable high forces on the calender bearings. The tendency ofpolyester polymers to hydrolyze during processing in the molten orsemi-molten state on rolls open to ambient conditions is also a concern.Typical PET polymers without the inclusion of process lubricants orinternal release additives, also have a tendency to stick to thecalendering rolls at typical processing temperatures.

Conventional processing of polyesters into film or sheet involvesextruding a polyester melt through a manifold of a flat die. Manual orautomatic die lip adjustment is used to control thickness across a webof material. Water-cooled chill rolls are used to quench the molten weband impart a smooth surface finish. A typical die extrusion process isshown in FIGS. 2A and 2B. Extrusion processes while producing film andsheet of excellent quality do not have the throughput and economicadvantages that are provided by calendering processes.

Thus, there exists a need in the art for an efficient and economicprocess to manufacture polyester films and sheets as an alternative toextrusion processes. Accordingly, it is to the provision of such thatthe present invention is primarily directed.

SUMMARY OF THE INVENTION

A polyester resin composition for calendering comprises a polyesterhaving a crystallization half time from a molten state of at least 5minutes and an additive for preventing sticking of the polyester tocalendering rolls. In another embodiment of the invention, a process forpreparing a film or a sheet comprises the step of calendering suchpolyester resin composition.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic of the polyester calendering process of thepresent invention.

FIG. 2A is a schematic of the polymer flow in a manifold of a flat dieutilized in prior art extrusion process for polyester film.

FIG. 2B is a schematic of a portion of the prior art extrusion processfor polyester film.

FIG. 3 is a graph showing roll-bearing force per time for Examples 2 and3.

FIG. 4 is a graph showing roll gap per time for Examples 2 and 3.

FIG. 5 is a graph showing roll torque per time for Examples 2 and 3.

DETAILED DESCRIPTION OF THE INVENTION

Certain amorphous or semi-crystalline polyester resin compositions areunexpectedly capable of being calendered using conventional calenderingprocesses to produce uniform films and sheets. The polyester resincompositions comprise a polyester having a crystallization half timefrom a molten state of about 5 minutes and an additive for preventingsticking to calendering rolls. The films and sheets typically have athickness in the range of about 2 mils (0.05 mm) to about 80 mils (2mm).

Polyesters useful in the practice of this invention include polyestershaving a crystallization half time from a molten state of at least about5 minutes, preferably about 12 minutes. The term “polyesters” as usedherein is meant to include copolyesters. Amorphous polyesters arepreferred because of their having a crystallization half time ofinfinity. Desired crystallization kinetics from the melt may also beachieved by adding polymeric additives or by altering the molecularweight characteristics of the polymer. An especially useful technique isto blend amorphous or very slow crystallizing polyester with the basepolyester.

Crystallization half times as defined by the present invention aremeasured using a Perkin-Elmer Model DSC-2 differential scanningcalorimeter. Each sample of 15.0 mg is sealed in an aluminum pan andheated to 290° C. at a rate of about 320° C./min for 2 minutes. Thesample is then cooled immediately to the predetermined isothermalcrystallization temperature at a rate of about 320° C./minute in thepresence of helium. The crystallization half time is determined as thetime span from reaching the isothermal crystallization temperature tothe point of a crystallization peak on the DSC curve.

Preferred polyesters comprise (i) at least 80 mole percent of a diacidresidue component selected from terephthalic acid,naphthalene-dicarboxylic acid, 1,4-cyclohexanedicarboxylic acid,isophthalic acid or mixtures thereof and (ii) at least 80 mole percentof a diol residue component selected from diols containing 2 to about 10carbon atoms and mixtures thereof. The diacid residue component is basedon 100 mole percent, and the diol residue component is based on 100 molepercent.

For the diacid residue component, any of the various isomers ofnaphthalenedicarboxylic acid or mixtures of isomers may be used, but the1,4, 1,5-, 2,6-, and 2,7-isomers are preferred. Also, cis, trans, orcis/trans isomer mixtures of 1,4-cyclohexanedicarboxylic acid may beused. Sulfoisophthalic acid may also be used. The diacid residuecomponent may be modified with minor amounts of up to about 20 molepercent of other diacids containing about 4 to about 40 carbon atoms andinclude succinic acid, glutaric acid, azelaic acid, adipic acid, subericacid, sebacic acid, dimer acid and the like.

For the diol residue component, the preferred diols include ethyleneglycol, diethylene glycol, neopentyl glycol, 1,4-cyclohexanedimethanoland mixtures thereof. More preferably, the diol residue component isfrom about 10 to 100 mole percent 1,4-cyclohexanedimethanol and fromabout 90 to 0 mole percent ethylene glycol. The diol residue componentmay also be modified with up to about 20 mole percent of other diols.Suitable modifying diols include 1,3-propanediol, 1,4-butanediol,1,5-pentanediol, 1,6-hexanediol, 1,8-octanediol,2,2,4-trimethyl-1,3-pentanediol, propylene glycol,2,2,4,4-tetramethyl-1,3-cyclobutanediol and the like.

The inherent viscosity (I.V.) of useful polyesters generally range fromabout 0.4 to about 1.5 dL/g, preferably about 0.6 to about 1.2 dL/g.I.V. as used herein refers to inherent viscosity determinations made at25° C. using 0.25 gram of polymer per 100 mL of a solvent composed of 60weight percent phenol and 40 weight percent tetrachloroethane.

The amorphous polyesters are made by melt phase techniques well known inthe art. The semicrystalline polyesters may be made by a combination ofmelt phase and solid phase poly-condensation procedures also well knownin the art.

In addition to the polyester, the polyester resin composition forcalendering also includes an additive that prevents the polyester fromsticking to the calendering rolls. The amount of additive used in thepolyester resin composition is typically about 0.01 to 10 weight percentbased on the total weight percent of the polyester resin composition.The optimum amount of additive used is determined by factors well knownin the art and considers variations in equipment, material, processconditions, and material film thickness.

Additives suitable for use in the present invention are well known inthe calendering art and include internal lubricants, slip agents ormixtures thereof. Examples of such additives include fatty acid amidessuch as erucylamide and stearamide; metal salts of organic acids such ascalcium stearate and zinc stearate; fatty acids and esters such asstearic acid, oleic acid, and palmitic acid; hydrocarbon waxes such asparaffin wax, polyethylene waxes, and polypropylene waxes; chemicallymodified polyolefin waxes; ester waxes such as carnauba; glycerol mono-and distearates; talc; and acrylic copolymers (for example, PARALOIDK175 available from Rohm & Haas). Antiblock and denest aids such asmicrocrystalline silica and erucylamide are also frequently used.

Conventional oxidative stabilizers may also be used with polyesters ofthe present invention to prevent oxidative degradation during processingof the molten or semi-molten material on the rolls. Suitable stabilizersinclude esters such as distearyl thiodipropionate or dilaurylthiodipropionate; phenolic stabilizers such as IRGANOX 1010 availablefrom Ciba-Geigy AG, ETHANOX 330 available from Ethyl Corporation, andbutylated hydroxytoluene; and phosphorus containing stabilizers such asIRGAFOS available from Ciba-Geigy AG and WESTON stabilizers availablefrom GE Specialty Chemicals. These stabilizers may be used alone or incombinations.

Sometimes the melt viscosity and the melt strength of the polyester areinsufficient for suitable processing on the calendering equipment. Inthese cases, the use of a melt strength enhancer is desirable such as bythe addition of small amounts (about 0.1 to about 2.0 mole %) of abranching agent to the polyesters either during their initialpreparation or during subsequent blending or feeding procedures prior toreaching the calendering equipment. Suitable branching agents includemultifunctional acids or glycols such as trimellitic acid, trimelliticanhydride, pyromellitic dianhydride, trimethylolpropane, glycerol,pentaerythritol, citric acid, tartaric acid, 3-hydroxyglutaric acid andthe like. These branching agents may be added directly to the polyesteror blended with the polyester in the form of a concentrate as describedin U.S. Pat. No. 5,654,347 and U.S. Pat. No. 5,696,176. It is alsopossible to use agents such as sulfoisophthalic acid to increase themelt strength of the polyester to a desirable level.

In addition to the additives described above, other additives typicallyused with polymers may be used as desired. These include plasticizers,dyes, colorants, pigments, fillers, matting agents, antiblocking agents,antistatic agents, chopped fibers, glass, impact modifiers, flameretardants, carbon black, talc, TiO₂ and the like.

In another embodiment of the present invention, a process for preparinga film or sheet comprises the step of calendering the polyester resincomposition described above. Conventional calendering processes andequipment are utilized to calender the polyester resin composition.Calenders having at least two adjacent heated rolls are suitable forprocessing the polyester resin composition, which is introduced betweenthe two rolls in pellet, powder or molten form. The rolls may be inseries or have a “L”, an inverted “L”, or a “Z” configuration. Typicalprocessing temperatures for the rolls will generally range from about130° C. to about 250° C., preferably about 140° C. to about 190° C.Predrying the polyester resin composition or venting excess moistureduring processing is preferred to prevent polymer degradation byhydrolysis.

With reference to FIG. 1, an inverted “L” configuration is used for thefour heated rolls 10, 12, 14, and 16. The four rolls form threecompressive nips or gaps. A feed nip 18 is formed between the first roll10 and second roll 12. A metering nip 20 is formed between the secondroll 12 and the third roll 14. A finishing nip 22 is formed between thethird roll 14 and the fourth roll 16. A hot strip or rod 24 of themolten polyester resin composition is uniformly fed by a pivot mountedfeeding device 26 into the feed nip 18. The molten composition ispreferably a homogeneous material as it exits the compounding orextruding operation (not shown). The molten composition may be furthermixed and heated by the circulating melt bank formed at the feed nip 18.The molten composition is eventually forced between the first roll 10and the second roll 12 by the rotating action of the rolls, then forcedthrough the metering nip 22 for reduction to its final desiredthickness, and finally forced through the finishing nip 22 to form afilm or sheet 28 of a particular gauge.

The resulting film or sheet 28 made from the polyester resin compositionof the present invention has a uniform thickness that is produced bypassing the polyester resin composition through the compressive nipsbetween the heated rolls. In effect, the polyester resin composition issqueezed between the nips which separates the rolls. Each successive nipbetween the calendering rolls reduces in opening size to obtain thefinal film or sheet gauge.

With reference to FIGS. 2A and 2B, the prior art of die extrusion forproducing polyester film or sheet utilizes a heated flat die 30. Apolyester melt supplied from a screw extruder (not shown) enters the die30 at the melt inlet 32. The melt is forced to flow uniformly across thewidth of the die 30 by an internal distribution manifold 34. Thisuniform flow must continue through the die land 36 and the exit plane38. The extruded web 40 of hot polymer is quenched on water-cooled rolls42. Final gauge control may be made by adjusting a die lip.

The present invention of calendering a polyester resin composition hassome significant advantages over extrusion of polyesters as a method offilm or sheet production. One significant advantage is the retention ofinherent viscosity after calendering as compared to prior tocalendering. As evident by the data in Table 2, the inherent viscosityof the polyester resin composition is retained at greater than 90percent, more preferably 95 percent.

Other advantages include high production rates, good thickness controland suitability for long, continuous production runs. For example,modern PVC calendering processes, which would be analogous to thepolyester calendering processes of the present invention, produceoutputs in excess of 3000 kg/hr and sheets having a thickness toleranceof +/−2% on 0.25 mm thick sheet. The sheets can have widths greater than2500 mm. This compares quite favorably over a typical sheet extruder forproducing polyester film or sheet. The typical extusion process hasoutputs of 500 to 750 kg/hr, has a thickness tolerance of +/−5% for a0.25 mm thick sheet and provides a sheet width of 1000 mm. The improvedconsistency of films or sheets made using the calendering process allowsfor less set up time and less heating and cycle process adjustmentsduring secondary forming operations. Economic advantages are alsoevident in terms of conversion cost per kg of sheet achieved by the highoutput calender processes over extrusion processes.

The present invention thus provides films and sheets made by calenderingthe polyester resin composition which have an excellent appearance andcan be used in a wide range of decorative and packaging applications.The films and sheets are readily thermoformed into various shapes forspecific packaging applications for both food and non-food products.They may be printed with a wide variety of inks and may be laminatedeither in-line or off-line with fabrics or other plastic films orsheets. Some specific end uses would include, graphic arts, transactioncards, security cards, veneers, wall coverings, book bindings, foldersand the like.

This invention can be further illustrated by the following examples ofpreferred embodiments thereof, although it will be understood that theseexamples are included merely for purposes of illustration and are notintended to limit the scope of the invention unless otherwisespecifically indicated.

EXAMPLES 1 to 7

Polyester compositions A and B are predried at 65° C. for 12 hours in adehumidified dryer and compounded with various additives as listed inTable 1, using a 30 mm Werner Pfleiderer 40:1 L/D co-rotating twin screwcompounding extruder.

TABLE 1 Material Id Ex. 1 Ex. 2 Ex. 3 Ex. 4 Ex. 5 Ex. 6 Ex. 7 PolyesterA¹ 100.0% 96.0%  — 97.0%  96.5%  99.0%  99.0%  Polyester B² — — 96.0%  —— — — PARALOID K175³ — 2.5% 2.5% 2.5% 2.5% — — IRGANOX — 0.5% 0.5% 0.5%— 0.5% 0.5% 1010/DSTDP⁴ MYVEROL 1806⁵ — 1.0% 1.0% — 1.0% — — ZnStearate⁶ — — — — — 0.5% — KENAMIDE S⁷ — — — — — — 0.5% ¹) Polyester A:Polyester containing an acid component of 100 mole % terephthalic acidand a glycol component of 31 mole % 1,4-cyclohexanedimethanol and 69mole % ethylene glycol ²) Polyester B: Polyester containing an acidcomponent of 100 mole % terephthalic acid and a glycol component of 3.5mole % 1,4-cyclohexanedimethanol and 96.5 mole % ethylene glycol. ³)PARALOID K175 is an acrylic-processing additive available from Rohm &Haas. ⁴) This is a mixture of IRGANOX 1010, which is a phenolicstabilizer available from Ciba-Geigy AG, and DSTDP, which is a distearylthiodipropionate commonly available in the industry. The mixture is 0.3%the former and 0.2% the latter. ⁵) MYVEROL 1806 is a glycerolmonostearate available from Eastman Chemical Company of Kingsport, TN,which is used as an internal lubricant ⁶) Zn Stearate is used as a slipadditive. ⁷) KENAMIDE S is a fatty acid amide available from WitcoCorporation, which is used as a slip additive.

The extruded compositions are then re-dried at 65° C. for 8 hrs andsealed in metal lined bags to prevent moisture absorption. Thecompositions are then calendered into films having a thickness of 0.2 mmusing an automated measuring roll mill available from Dr. Collin Gmbh ofEbersberg, Germany at a set roll temperature of 165° C. Bearing forceexerted on the rolls, roll torque, calenderability, weight averagemolecular weight, and polymer crystallinity are measured and summarizedin Table 2 and FIGS. 3 to 5.

TABLE 2 Material Id Ex. 1 Ex. 2 Ex. 3 Ex. 4 Ex. 5 Ex. 6 Ex. 7 IV of 0.70.75 0.74 0.73 0.75 0.73 0.71 Compounded Pellets IV after — 0.74 0.73 —0.74 0.74 — Calendering Weight average — 40824 — — 40824 37857 —molecular wt by GPC of Pellets Weight average — 40163 — — 40054 37792 —molecular wt by GPC of calendered sheet % Crystallinity- — — 10.8 — — —— Pellets % Crystallinity- — 0 31.7 — 0 0 — Calendered CalenderabilityNo* Yes No** No* Yes Yes No* *Excessive sticking to the calender rolls -not possible to strip the film **Crystallization in the calender rollsprevents formation of a molten film

FIG. 3 shows the role bearing force per time for Examples 2 and 3. Thecomposition of Example 2 exhibited good calenderability evidenced by thestable roll bearing force over time. Example 2 had a crystallizationhalf time of infinity. The composition of Example 3 readily crystallizedbetween the heated rolls resulting in a high force being exerted on theroll bearings, thus being unsuitable for calendering. Example 3 had acrystallization half time of less than 5 minutes.

FIG. 4 shows roll gap per time for Examples 2 and 3. Example 2calendered at a set roll gap. However, Example 3 had morphology changesassociated with crystallization, which created forces causing the rollsto separate.

FIG. 5 shows roll torque per time for Examples 2 and 3. Example 2 hadconsistent roll torque and uniform calender behavior. Example 3 hadinconsistent torque prior to discharge from the rolls.

Examples 1-7 demonstrated the feasibility of calendering polyesters.Polyester A was an amorphous polyester having a crystallization halftime of infinity. Polyester B has a crystallization half time of lessthan 5 minutes and was not readily calendered.

EXAMPLE 8

Using the compounding procedure of Ex. 1, a polyester containing an acidcomponent of 100 mole % terephthalic acid and a glycol component of 12mole % 1,4-cyclohexanedimethanol and 88 mole % ethylene glycol (I.V. of0.74) was compounded with 1.0 weight % zinc stearate and 1.0 weight %MYVEROL 1806. The material was heated to its molten state of 260° C. andthen transferred to a hot roll mill calender. The copolyester wascalendered through compressive nips on the calender rolls to a finalsheet thickness of 0.65 mm. This example demonstrated the feasibility ofcalendering a polyester having a crystallization half time of 12minutes.

I claim:
 1. A calendered sheet made by a process comprising the step ofcalendering a polyester resin composition comprising (a) a polyesterwhich comprises (i) a terephthalic acid residue component and (ii) adiol residue component comprising ethylene glycol and1,4-cyclohexanedimethanol, and (b) a fatty ester.
 2. A calendered sheetaccording to claim 1, wherein said polyester resin composition comprises(a) a polyester which comprises (i) a terephthalic acid residuecomponent and (ii) a diol residue component which is 69 mole % ethyleneglycol and 31 mole % 1,4-cyclohexanedimethanol, and (b) a fatty ester.3. A calendered sheet according to claim 2, wherein said polyester resincomposition comprises glycerol monostearate.
 4. A calendered sheetaccording to claim 3, wherein said polyester resin composition comprises1 wt % glycerol monostearate, based on the total wt % of the polyesterresin composition.
 5. A transaction card or a security card formed froma calendered sheet according to claim 2.